Undrained Fragility of Clean Sands, Silty Sands, and Sandy Silts

In this paper, intergranular $\left(e_c\right)$ and interfine $\left(e_f\right)$ void ratios and confining stress are used as indices to characterize the stress–strain response of gap graded granular mixes. It was found that at the same global void ratio (e) and confining stress, the collapse potential (fragility) of silty sand increases with an increase in fines content (FC) due to a reduction in intergranular contact between the coarse grains. Beyond a certain threshold fines content $\left({\mathrm{FC}}_{\mathrm{th}}\right)\text{,}$ with further addition of fines, the interfine contact friction becomes significant. The fragility decreases and the soil becomes stronger. The value of ${\mathrm{FC}}_{\mathrm{th}}$ depends on e and the characteristics of fines and coarse grains. At $\mathrm{FC}<{\mathrm{FC}}_{\mathrm{th}}\text{,}$ intergranular contact friction plays the primary role. It is postulated that fines either provide a beneficial secondary cushioning effect or contribute to fragility, depending on the nature of the soil's matrix structure and the magnitude of $e_c\text{.}$ At the same $e_c\text{,}$ the fines that fall within the intergranular voids provide a cushioning effect and slightly reduce the fragility. When fines fall between some of the coarse grains and partially support the coarse grain skeleton, the soil is very fragile. The contribution to fragility is dominant when the soil is very loose in terms of $e_c\text{.}$ At large fines contents $\left(\mathrm{FC}>{\mathrm{FC}}_{\mathrm{th}}\right)\text{,}$ fine grain friction plays a primary role and dispersed coarse grains provide a beneficial, secondary reinforcement effect. At the same $e_f\text{,}$ the collapse potential decreases with an increase in sand content. Beyond a certain limiting fines content, the soil behavior is controlled by $e_f$ only. An intergranular matrix diagram is presented that delineates zones of different behaviors of granular mixes as a guideline to determine the anticipated behavior of gap-graded granular mixes. New equivalent intergranular contact void ratios, $(e_c{)}_{\mathrm{eq}}$ and $(e_f{)}_{\mathrm{eq}}\text{,}$ are introduced to characterize the behavior of such soils, at $\mathrm{FC}<{\mathrm{FC}}_{\mathrm{th}}$ and $\mathrm{FC}>{\mathrm{FC}}_{\mathrm{th}}\text{,}$ respectively.